Delay demodulation devices

ABSTRACT

A delay demodulation device, which reduces chip size and polarization dependent frequency (PDf), is provided. The delay demodulation device comprises: an input waveguide, which receives DQPSK signals; a Y-shape waveguide, which splits the input waveguide; a first Mach-Zehnder interferometer; and a second Mach-Zehnder interferometer. Both end of two arm waveguides of first Mach-Zehnder interferometer and both ends of two arm waveguides of second Mach-Zehnder interferometer are angled toward the center portion of a Planar Lightwave Circuit (PLC). Because of the angle, the length of the two arm waveguides of the first Mach-Zehnder interferometer and the length of the two arm waveguides of the second Mach-Zehnder interferometer in Z-direction can be shortened, and input couplers and output couplers of the Mach-Zehnder interferometers in Z-direction can be shortened as well. The area occupied by the Mach-Zehnder interferometers is also reduced.

TECHNICAL FIELD

The present invention relates to delay demodulation devices used for optical fiber communication, and particularly, relates to delay demodulation devices equipped with Planar Lightwave Circuit (PLC) type Mach-Zehnder interferometers, which demodulate Differential Quadrature Phase Shift Keying (DQPSK) signals in Dense Wavelength Division Multiplexing (DWDM) transmission.

RELATED ARTS

Recently, with the rapid growth in broadband networks, high speed optical transmission systems (toward transmission rate of 40 Gbps) are being investigated actively. However, when the transmission rate is increased, the time duration per 1 bit of optical signal is reduced and, because of the characteristics of an optical fiber, wave shape is deteriorated, and therefore the quality of a communication line is deteriorated. For 40 Gbps-class long distance transmission, transponders that transform an optical signal to an electrical signal and then transform the electrical signal back to an optical signal are needed in the transmission path. Therefore, it is difficult to create a high speed optical transmission system using existing optical fiber networks.

Because of this issue, research and development has been done in Differential Quadrature Phase Shift Keying (DQPSK). DQPSK reduces deterioration of signal-wave profile by increasing the time duration per bit of an optical signal.

DQPSK transmits four symbols of information as four corresponding optical phase shifts. In other words, each symbol of information corresponds to a value (0, 1, 2 or 3), which comprises two bits of data, and the symbols of information are transmitted by shifting the phase of carrier wave between adjacent symbols by an amount (θ, θ+π/2, θ+π or θ+3π/2) determined by the pair of bits to be transmitted. 40 Gbps DQPSK transmission can transmit four times longer distance than conventional 40 Gbps transmission. Because of DQPSK, it is believed that construction of networks between large cities can be achieved using existing optical fiber networks.

For example, conventional delay demodulation devices, which demodulate in receiving devices by using DQPSK signals or Differential Phase Shift Keying (DPSK) signals, are disclosed in Japanese Patent Application Laid-open 2007-60442, and in Japanese Patent Application Laid-open 2007-151026.

Photo receiving circuits disclosed in Japanese Patent Application Laid-open 2007-60442 are equipped with Mach-Zehnder interferometers, which propagate return-to-zero (RZ) modulated DPSK signals through a pair of optical paths, which are equipped with a one-symbol delay element in one of the pair optical paths.

Also, demodulation devices disclosed in Japanese Patent Application Laid-open 2007-151026 use Michelson interferometers to demodulate DPSK or DQPSK optical signals.

In delay detection of 40 Gbps DQPSK transmission, two Mach-Zehnder interferometers and a Planar Lightwave Circuit (PLC)-type delay circuit (i.e. delay demodulation device), which demodulates DQPSK signals, are used. In the 40 Gbps DQPSK transmission, the permissible value of Polarization Dependent frequency (PDf) in the delay circuit is said to be less than 0.2 GHz. As a way to reduce the PDf, a half wave plate can be inserted in the Mach-Zehnder interferometers. However, it is difficult to lower the PDf to be less than 0.2 GHz by just inserting the half wave plate. Also, inserting the half wave plate causes yield ratio as well. Because of the above reasons, it is very difficult to manufacture delay demodulation circuits with small PFf (<0.2 GHz) consistently. Also, size reduction and reduction in power consumption of optical fiber modules containing the demodulation devices are also desired.

BRIEF SUMMARY OF THE INVENTION

The present invention seeks to overcome the above-identified problems. The purpose of the present invention is to provide delay demodulation devices with reduced Polarization Dependent frequency and reduced chip size.

To solve the above issue, a Planar Lightwave Circuit (PLC)-type delay demodulation device, which demodulates Differential Quadrature Phase Shift Keying (DQPSK) signal, is invented. The delay demodulation device comprises: (i) an input waveguide, which receives the DQPSK signals; (ii) a Y-shape waveguide, which splits the input waveguide; and (iii) first and second Mach-Zehnder interferometers. The first Mach-Zehnder interferometer comprising: an input coupler, which is connected to one of the two waveguides split by the Y-shape waveguide; an output coupler, which is connected to output waveguides; and two arm waveguides having different lengths and connected between the input coupler and the output coupler. The second Mach-Zehnder interferometer comprising: an input coupler, which is connected to the other waveguide split by the Y-shape waveguide; an output coupler, which is connected to output waveguides; and two arm waveguides having different lengths and connected between the input coupler and the output coupler, wherein the first Mach-Zehnder interferometer and the second Mach-Zehnder interferometer cross within the same area.

According to the construction above, the PLC can be made smaller, particularly because areas including the two arm waveguides in the first and second Mach-Zehnder interferometers are smaller, the PLC chip can be made smaller.

By making the chip smaller, temperature distribution along the PLC surface evens out, and shifts in center wavelengths due to environment and temperature fluctuation are very small. Also, because of the smaller chip size, stress distribution within the chip, which causes birefringence, is reduced, and shifts in center wavelengths due to the environment and temperature fluctuation can be made very small. Therefore, there are practically no wavelength shifts due to environment and temperature fluctuation, and the delay demodulation devices with small initial PDf can be created. Furthermore, by reducing the chip size, optical fiber modules with delay demodulation devices can be smaller, and the power consumption of the modules can be reduced as well.

According to another a Planar Lightwave Circuit (PLC)-type delay demodulation device, the two arm waveguides of the first Mach-Zehnder interferometer and the two arm waveguides of the second Mach-Zehnder interferometer are placed in the same areas such that the two arm waveguides of the first Mach-Zehnder interferometer and the two arm waveguides of the second Mach-Zehnder interferometer cross each other. Because of the construction, the PLC chip can be reduced in size and in PDf.

According to another a Planar Lightwave Circuit (PLC)-type delay demodulation device, the first Mach-Zehnder interferometer and the second Mach-Zehnder interferometer are placed on a PLC base plate such that the first Mach-Zehnder interferometer and the second Mach-Zehnder interferometer are bilaterally-symmetric to each other. Because of the construction, the PLC chip can be further reduced in size and in PDf.

According to another a Planar Lightwave Circuit (PLC)-type delay demodulation device, a half-wave plate is inserted at the center portion of the two arm waveguides of the first Mach-Zehnder interferometer and at the center portion of the two arm waveguides of the second Mach-Zehnder interferometer. Because of the construction, the PLC chip can be reduced in PDf.

According to another a Planar Lightwave Circuit (PLC)-type delay demodulation device, the center portion of the two arm waveguides of the first Mach-Zehnder interferometer are parallel to and close to each other, and the center portion of the two arm waveguides of the second Mach-Zehnder interferometer are also parallel to and close to each other. Because of the construction, retardation by the half wave plate can be suppressed.

According to another a Planar Lightwave Circuit (PLC)-type delay demodulation device, two individual ends of the output waveguides connected to the output coupler of the first Mach-Zehnder interferometer and two individual ends of the output waveguides connected to the output coupler of the second Mach-Zehnder interferometer are placed on the same side of a PLC chip. Because of the construction, both the PLC and the PLC chip can be further reduced in size.

According to another a Planar Lightwave Circuit (PLC)-type delay demodulation device, the PLC chip is approximately in square planar shape.

According to another a Planar Lightwave Circuit (PLC)-type delay demodulation device, at least one heater is placed on at least one of the two arm waveguides of the first Mach-Zehnder interferometer, and at least one heater is placed on at least one of the two arm waveguides of the second Mach-Zehnder interferometer.

According to the construction above, the PDf can be adjusted by using the heaters on either the first or the second Mach-Zehnder interferometer. After the adjustment, phase shift control (phase shift trimming) can be performed by using heaters on one of the two Mach-Zehnder interferometers to shift the phase of one Mach-Zehnder interferometer by π/2 radians to the phase of the other Mach-Zehnder interferometer.

The present invention can be provided to delay demodulation devices with reduced Polarization Dependent frequency and reduced chip size.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the invention will appear more fully hereinafter from a consideration of the following description taken into connection with the accompanying drawing wherein one example is illustrated by way of example, in which;

FIG. 1 is a plan view of a delay demodulation device in one embodiment of the present invention.

FIG. 2 is a block diagram of an optical transmission system with Differential Quadrature Phase Shift Keying (DQPSK).

FIG. 3 is a cross-sectional drawing taken along line X-X in FIG. 1.

FIG. 4 is a cross-sectional drawing taken along line Y-Y in FIG. 1.

FIG. 5 is a graph showing a spectrum of the delay demodulation device disclosed in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Detailed description as follows;

Construction of the delay demodulation device is shown in FIG. 1 through FIG. 5.

FIG. 1 is a plan view of a delay demodulation device in one embodiment of the present invention, FIG. 2 is a block diagram of an optical transmission system with Differential Quadrature Phase Shift Keying (DQPSK). FIG. 3 is a cross-sectional drawing taken along line X-X in FIG. 1, FIG. 4 is a cross-sectional drawing taken along line Y-Y in FIG. 1. FIG. 5 is a graph showing a spectrum of the delay demodulation device disclosed in FIG. 1.

Delay demodulation device 1 shown in FIG. 1 is a Planar Lightwave Circuit (PLC)-type delay demodulation device, which demodulates Differential Quadrature Phase Shift Keying (DQPSK) signals. Delay demodulation device 1 is, for example, a 40 Gbps DQPSK delay demodulation device used in a 40 Gbps DQPSK optical transmission system shown in FIG. 2.

In the optical transmission system, DQPSK signals are transmitted from an optical transmitter 40 to an optical fiber transmission line 54. In DQPSK signals, each symbol of information corresponds to a value (0, 1, 2 or 3), which comprises two bits of data, and the symbols of information are transmitted by shifting the phase of carrier wave between adjacent symbols by an amount (θ, θ+π/2, θ+π or θ+3π/2) determined by the pair of bits to be transmitted. DQPSK signals transmitted from the optical fiber transmission line 54 to an optical receiver 50 are converted to optical signals with four different intensities by the delay demodulation device 1, and furthermore, the optical signals are converted to electric signals by balanced receivers 51 and 52. In a receiving electric circuit 53, various processes such as decryption process are performed.

Delay demodulation device 1 shown in FIG. 1 comprises an input waveguide 2, which receives DQPSK signals; a Y-shape waveguide 3, which splits the input waveguide 2; a first Mach-Zehnder interferometer 4; and a second Mach-Zehnder interferometer 5.

The first Mach-Zehnder interferometer 4 comprises: an input coupler 6 connected to one of the two waveguides 14, 15 (in FIG. 1, it is waveguide 14), which are split by the Y-shape waveguide 3; an output coupler 7 connected to output ends of two output waveguides 21, 22; and two arm waveguides 8, 9, which are connected between the both couplers 6, 7. The two arm waveguides 8, 9 are different in lengths. Similarly, the second Mach-Zehnder interferometer 5 comprises: an input coupler 10 connected to the other waveguides (in FIG. 1, it is waveguide 15) of the two waveguides 14, 15, which are split by the Y-shape waveguide 3; an output coupler 11, which is connected to output ends of two output waveguides 23, 24; and two arm waveguides 12, 13, which are connected between the both couplers 10, 11. The two arm waveguides 12, 13 are different in lengths.

The input couplers 6, 10 and the output couplers 7, 11 are 2 inputs×2 outputs-type, 3 dB couplers (50% directional couplers). One end of the input coupler 6 of the first Mach-Zehnder interferometer 4 is connected to the waveguide 14, and one end of the input coupler 10 of the second Mach-Zehnder interferometer 5 is connected to the waveguide 15.

Also, the two output ends (a through port and a cross port) of the output coupler 7 of the first Mach-Zehnder interferometer 4 are connected to the first and second output waveguides 21, 22, respectively. In a similar fashion, the two output ends (a through port and a cross port) of the output coupler 11 of the second Mach-Zehnder interferometer 5 are connected to the third and fourth output waveguides 23, 24, respectively.

Also, in the two arm waveguides 8, 9 of the first Mach-Zehnder interferometer 4, there is a difference in length of the waveguides ΔL to make phase shift of the DQPSK signal in one end (i.e. the arm waveguide 8) delay by π radians against the other (i.e. the arm waveguide 9). In a similar fashion, in the two arm waveguides 12, 13 of the second Mach-Zehnder interferometer 5, there is a difference in length of the waveguides ΔL to make the phase shift of the DQPSK signal in one end (i.e. arm waveguide 12) delay by π radians against the other (i.e. arm waveguide 13).

Characteristics of the delay demodulation device 1, which relate to an embodiment of the present invention, is that on a PLC 1A, the two arm waveguides 8, 9 of the first Mach-Zehnder interferometer 4, and the two arm waveguides 12, 13 of the second Mach-Zehnder interferometer 5 are overlapped in the same areas.

In one of the embodiments of the present invention, as an example of overlapping, the arm waveguides 8, 9 and the arm waveguides 12, 13 are placed on the same areas of the PLC 1A such that the arm waveguides 8, 9 and the arm waveguides 12, 13 cross each other four times.

In other words, as shown in FIG. 1, the arm waveguide 8 of the first Mach-Zehnder interferometer 4 crosses with the arm waveguide 12 of the second Mach-Zehnder interferometer 5 at a crossover point 61, with the arm waveguide 13 of the second Mach-Zehnder interferometer 5 at a crossover point 62, with the arm waveguide 13 at a crossover point 63, and with the arm waveguide 12 at a crossover point 64.

The arm waveguide 9 of the first Mach-Zehnder interferometer 4 crosses with the arm waveguide 12 of the second Mach-Zehnder interferometer 5 at a crossover point 65, with the arm waveguide 13 of the second Mach-Zehnder interferometer 5 at a crossover point 66, with the arm waveguide 13 at a crossover point 67, and with the arm waveguide 12 at a crossover point 68.

In a similar fashion, the arm waveguide 12 of the second Mach-Zehnder interferometer 5 crosses with the arm waveguide 9 of the first Mach-Zehnder interferometer 5 at the crossover point 65, with the arm waveguide 8 of the first Mach-Zehnder interferometer 5 at the crossover point 61, with the arm waveguide 8 at the crossover point 64, and with the arm waveguide 9 at the crossover point 68.

Then, the arm waveguide 13 of the second Mach-Zehnder interferometer 5 crosses with the arm waveguide 9 of the first Mach-Zehnder interferometer 5 at the crossover point 66, with the arm waveguide 8 of the first Mach-Zehnder interferometer 5 at the crossover point 62, with the arm waveguide 8 at the crossover point 63, and with the arm waveguide 9 at the crossover point 67.

In each of the crossover point 61˜68, where two arm waveguides cross, the optical signal after the cross over point propagates in the same arm waveguide, which the signal was propagated before. For example, at the crossover point 61, where two arm waveguides 8, 12 cross, the optical signal propagates in the arm waveguide 8 continues to propagate in the arm waveguide 8 after the crossover point 61. In a similar way, the optical signal propagates in the arm waveguide 12 continues to propagate in the arm waveguide 12 after the crossover point 61

PLC 1A shown in FIG. 1 has the input waveguide 2, the Y-shape waveguide 3, the first Mach-Zehnder interferometer 4, the second Mach-Zehnder interferometer 5, and the four output waveguides 21˜24 made all from silica glass. The delay demodulation device 1 comprising the PLC 1A is manufactured as follow.

With flame hydrolysis deposition (FHD), silica material (SiO₂-type glass particles), which makes a lower cladding layer and a core layer, is deposited on a PLC base plate 30 (such as a silica base plate) as shown in FIG. 3. Then, a glass coating made by the deposition is fused (and becomes transparent) by adding heat. Later, desired waveguides are created by photo lithography and reactive ion etching, and a upper cladding is created with FHD method. In FIG. 3, on the PLC base plate 30, a cladding layer is created by the lower cladding layer and the upper cladding layer, and the arm waveguides 8, 12 are created as the core layer inside of the cladding layer 31. The PLC base plate 30 is approximately in square planar shape as shown in FIG. 1.

In the delay demodulation devices related to the present invention, the first Mach-Zehnder interferometer 4 and the second Mach-Zehnder interferometer 5 are placed symmetrical to each other on the PLC base plate 30.

Also, to reduce PDf, a half wave plate 47 is inserted at the center portion of the two arm waveguides 8, 9 of the first Mach-Zehnder interferometer 4 and at the center portion of the two arm waveguides 12, 13 of the second Mach-Zehnder interferometer 5 of the delay demodulation device 1. As shown in FIG. 4, a groove 49 is created on the cladding layer 31 to insert the half wave plate 47. The groove 49 is tilted by 8° to make the half-wave plate 47 tilt by 8° as shown in FIG. 4, to prevent loss due to reflections by the half-wave plate 47.

Also, as shown in FIG. 1, in the delay demodulation device 1, the center portion of the two arm waveguides 8, 9 of the first Mach-Zehnder interferometer 4 are parallel to and close to each other, and the center portion of the two arm waveguides 12, 13 of the second Mach-Zehnder interferometer 5 are also parallel to and close to each other.

The other characteristics of the delay demodulation device 1 are as follows. As shown in FIG. 1, ends of the input waveguide 2, the two output waveguides 21, 23, and the other two output waveguides 23, 24 appear on the same side 1 a of the PLC chip 1B, which is approximately square planar in shape. In other words, the ends of the waveguides 2 and the four output waveguides 21˜24 appear on the same side 1 a (one of four sides) of the PLC chip 1B, and are close to each other.

Also, in the delay demodulation device 1, heaters are placed on the two arm waveguides 8, 9 of the first Mach-Zehnder interferometer 4, and the two arm waveguides 12, 13 of the second Mach-Zehnder interferometer 5.

In an embodiment of the present invention, as an example, heaters A, C are placed on the both sides of the center portion of the arm waveguide 8; and heaters B, D are placed on the both sides of the center portion of the arm waveguide 9. In a similar way, heaters E, G are placed on the both sides of the center portion of the arm waveguide 12, and heaters F, H are placed on the both sides of the center portion of the arm waveguide 13. Heaters A˜H are placed above the corresponding arm waveguide, and the heaters are Tantalum compound thin film heaters made by a weld slag, which are placed onto the upper cladding (the cladding layer 31 in FIG. 3). In FIG. 3, the heaters A, E placed above the cladding layer 31 of the arm waveguides 8, 12 are shown.

Also, in the delay demodulation device 1, the output ends of output waveguides 21, 22 are output ports (a first output port and a second output port), which output signals 1, 2 wherein the phase of one output signal is shifted by π radians with respect to the other (see FIG. 5). In a similar way, the output ends of output waveguides 23, 24 are output ports (a third output port and a fourth output port), which output signals 3 and 4 wherein the phase of one output signal is shifted by π radians with respect to the other (see FIG. 5).

In the delay demodulation device 1, DQPSK signal transmitted from the optical fiber transmission line 54 to the optical receiver 50 splits by the Y-shape waveguide 3, and the split DQPSK signals propagate to the two arm waveguides 8 (wherein the lengths are different) of the first Mach-Zehnder interferometer. The Mach-Zehnder interferometer 4 shifts the phase of the DQPSK signal transmitted in one arm waveguide 8 by one symbol (i.e. π radians) with respect to the phase of the signal in the other arm waveguide 9. Similarly, the second Mach-Zehnder interferometer 5 shifts the phase of the DQPSK signal transmitted in one arm waveguide 12 by one symbol (i.e. π radians) with respect to the phase of the signal in the other arm waveguide 13.

The delay demodulation device 1 adjusts PDf, for example, by using the heaters A, C or the heaters B, D of the Mach-Zehnder interferometer 4. After the adjustment, the delay demodulation device 1 performs phase shift control (or phase shift trimming) to shift the phase of one Mach-Zehnder interferometer by π/2 radians to the phase of the other Mach-Zehnder interferometer, for example, by using the heaters A and C.

EMBODIMENTS

Delay demodulation device 1 has a PLC 1A on a silica base plate 30 shown in FIG. 3. The PLC 1A comprises: an input waveguide 2; a Y-shape waveguide 3; Mach-Zehnder interferometers 4, 5; and output waveguides 21˜24, wherein all of the components are made from silica glass. To create the demodulation device 1 FHD method, photo lithography, and reactive ion etching are used.

In the manufactured delay demodulation device 1, the difference in the refractive indexes between the cladding layer and the core layer (specific refractive index difference Δ) is 1.5%, and the size of the circuit (i.e. PLC chip 1B) is relatively small (i.e. 19 mm by 16 mm). Its free spectral range (FSR) is 20 GHz. The PDf is adjusted by using heaters on one of the two Mach-Zehnder interferometers 4, 5. After the adjustment, phase shift control (or phase shift trimming) is performed by using heaters on one of the two Mach-Zehnder interferometers 4, 5 to shift the phases of one Mach-Zehnder interferometer by π/2 radians with respect to the phase of the other Mach-Zehnder interferometer.

To create a packaging, a fiber array comprising four optical fibers in a line is connected to one side 1 a of the PLC chip 1B. The side 1 a has the ends (i.e. output ports) of output waveguides 21˜24, which output optical signals to the outputs 1˜4, respectively. Also, as a temperature control device, a peltier element and a thermostat are used. Then, an optical fiber module having the delay demodulation device 1 is manufactured.

FIG. 5 shows the results of the optical characteristics of the 40 Gbps DQPSK delay demodulation device 1 (i.e. PLC type demodulation Mach-Zehnder interferometers delay circuit for DQPSK signal). Insertion loss of less than 6 dB and extremely low PDf (less than 0.1 GHz) are achieved.

According to the embodiment presented above, the following advantages can be achieved.

In the delay demodulation device 1, the two arm waveguides 8, 9 of the first Mach-Zehnder interferometer 4, and the two arm waveguides 12, 13 of the second Mach-Zehnder interferometer 5 are overlapped in the same areas within the PLC 1A. More specifically, the arm waveguides 8, 9 and the arm waveguides 12, 13 are placed on the same areas of the PLC 1A such that the arm waveguides 8, 9 and the arm waveguides 12, 13 cross each other four times. Because of such construction, the PLC 1A can be made smaller. In particular, because areas including the two arm waveguides 8, 9 in the first Mach-Zehnder interferometer 4 and two arm waveguides 12, 13 in the second Mach-Zehnder interferometer 5 are smaller, the chip (i.e. PLC chip) 1B can be made smaller as well.

By making the PLC chip 1B smaller, temperature distribution within the PLC surface 1A improves and shifts in the center wavelengths due to environment and temperature fluctuation can be made very small.

Also, by making the PLC chip smaller, stress distribution within the PLC chip 1B, which causes birefringence, is reduced and shifts in the center wavelengths due to the environment and temperature fluctuation can be made very small. Therefore, there is little or no wavelength shift due to the environment and temperature fluctuation, and the delay demodulation devices with small initial PDf can be made.

By making the PLC chip smaller, the optical fiber modules with delay demodulation devices can be made smaller, and power consumption can be reduced.

Because the arm waveguides 8, 9 and the arm waveguides 12, 13 are placed within the same area of the PLC 1A (the arm waveguides 8, 9 and the arm waveguides 12, 13 cross each other four times), the PLC chip 1B can be made smaller and achieve low PDf.

Because the first Mach-Zehnder interferometer 4 and the second Mach-Zehnder interferometer 5 are placed symmetrically on the PLC base plate 30, the PLC chip 1B can be further reduced in size and in PDf.

Because the half wave plate 47 is inserted at the center of the two arm waveguides 8, 9 of the first Mach-Zehnder interferometer 4 and at the center of the two arm waveguides 12, 13 of the second Mach-Zehnder interferometer 5 of the delay demodulation device 1, the PDf can be reduced.

The center portions of the two arm waveguides 8, 9 of the first Mach-Zehnder interferometer 4 are placed in parallel and close to each other. The center portions of the two arm waveguides 12, 13 of the second Mach-Zehnder interferometer 5 are placed in parallel and close to each other. Because of the construction, retardation by the half wave plate 47 can be suppressed.

Because the ends of the input waveguide 2 and four output waveguides 21˜24 face on the same side 1 a of the PLC chip 1B, the PLC chip 1B can be further reduced in size.

Because the heaters A˜H are placed on the arm waveguides of the first and second Mach-Zehnder interferometers 4, 5, the PDf can be adjusted by using the heaters on either the first or the second Mach-Zehnder interferometer. After the adjustment, phase shift control (phase shift trimming) can be performed by using heaters on one of the two Mach-Zehnder interferometers 4, 5 to shift the phase of one Mach-Zehnder interferometer by π/2 radians to the phase of the other Mach-Zehnder interferometer.

In the above embodiment, because the arm waveguides 8, 9 and the arm waveguides 12, 13 cross each other four times, there are some transmission losses at crossover points 61˜68. However, the total transmission loss is relatively small (i.e. 0.1˜0.2 dB).

Also, in the above embodiment, the arm waveguides 8, 9 and the arm waveguides 12, 13 cross each other four times. However, the present invention can be applied to delay demodulation devices, which cross two arm waveguides of a first Mach-Zehnder interferometer and two arm waveguides of a second Mach-Zehnder interferometer twice.

Also, in the above embodiment, as a preferred embodiment, the center portions of the two arm waveguides 8, 9 of the first Mach-Zehnder interferometer 4, and the two arm waveguides 12, 13 of the second Mach-Zehnder interferometer 5 are placed adjacent to each other. However, without depending of such construction of the embodiment, the present invention can be applied to delay demodulation devices, whose center portions of two arm waveguides of a first Mach-Zehnder interferometer, and the center portions of two arm waveguides of a second Mach-Zehnder interferometer can be placed apart from and parallel to each other.

The present invention is not limited to the above described embodiments and various and modifications may be possible without departing from the scope of the present invention. 

1. A Planar Lightwave Circuit (PLC)-type delay demodulation device for demodulating Differential Quadrature Phase Shift Keying (DQPSK) signal, the delay demodulation device comprising: an input waveguide, which receives the DQPSK signals; a Y-shape waveguide, which splits the input waveguide; a first Mach-Zehnder interferometer comprising: an input coupler, which is connected to one of the two waveguides split by the Y-shape waveguide; an output coupler, which is connected to output waveguides; and two arm waveguides having different lengths with respect to each other and connected between the input coupler and the output coupler, and a second Mach-Zehnder interferometer comprising: an input coupler, which is connected to the other waveguide split by the Y-shape waveguide; an output coupler, which is connected to output waveguides; and two arm waveguides having different lengths with respect to each other and connected between the input coupler and the output coupler, wherein the first Mach-Zehnder interferometer and the second Mach-Zehnder interferometer cross each other within the same areas.
 2. The delay demodulation device of claim 1, wherein the two arm waveguides of the first Mach-Zehnder interferometer and the two arm waveguides of the second Mach-Zehnder interferometer are placed in the same areas such that the two arm waveguides of the first Mach-Zehnder interferometer and the two arm waveguides of the second Mach-Zehnder interferometer cross each other.
 3. The delay demodulation device of claim 1, wherein the first Mach-Zehnder interferometer and the second Mach-Zehnder interferometer are placed on a PLC base plate such that the first Mach-Zehnder interferometer and the second Mach-Zehnder interferometer are bilaterally-symmetric to each other.
 4. The delay demodulation device of claim 1, wherein a half-wave plate is inserted at the center portion of the two arm waveguides of the first Mach-Zehnder interferometer and at the center portion of the two arm waveguides of the second Mach-Zehnder interferometer.
 5. The delay demodulation device of claim 4, wherein the center portion of the two arm waveguides of the first Mach-Zehnder interferometer are parallel to and close to each other, and the center portion of the two arm waveguides of the second Mach-Zehnder interferometer are also parallel to and close to each other.
 6. The delay demodulation device of claim 1, two individual ends of the output waveguides connected to the output coupler of the first Mach-Zehnder interferometer and two individual ends of the output waveguides connected to the output coupler of the second Mach-Zehnder interferometer are placed on the same side of a PLC chip.
 7. The delay demodulation device of claim 6, wherein the PLC chip is approximately in square planar shape.
 8. The delay demodulation device of claim 1, wherein at least one heater is placed on at least one of the two arm waveguides of the first Mach-Zehnder interferometer, and at least one heater is placed on at least one of the two arm waveguides of the second Mach-Zehnder interferometer. 